Tuberculosis in the Triassic

Very few infectious diseases leave their traces on your bones. This is a problem for palaeontologists because only bones tend to get fossilised, and soft tissues are vanishing rare in the geological record. Even when these are preserved they’re extremely unlikely to reveal signs of any infection the animal might have suffered. Therefore very few infection diseases actually have much in the way of a fossil record. I’ve written about a couple of exceptions here before, namely leprosy and syphilis, both of which leave very distinctive traces on a skeleton, but there is another important human disease that may have an even more venerable fossil record: tuberculosis.

In 2018 the fossil remains of a Triassic-aged, marine reptile were re-examined using a technique called X-ray microtomography scanning (XMT). Put extremely simply this technique involves taking numerous X-rays of the bone while it is rotated, allowing a computer to build up a 3D model of the bone’s interior and exterior. This allows researchers to get a really good, close up view of the microstructures of the bones, in this case part of the rib cage. What they found were “bleb-like” structures in the outer membrane of the bones. These lesions were tiny, ranging from 0.5-3mm across, smooth and roughly elliptical, and they stood out to the palaeontologists who examined them because they resembled a rare effect seen in the bones of human patients suffering from chronic tuberculosis.

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Mycobacterium tuberculosis. Source: Wikimedia Commons

Tuberculosis (often shorted to TB) is a bacterial disease spread through tiny droplets in the coughs and sneezes of an infected patient. It infects the lungs and can cause a range of symptoms including a persistent cough, fevers, tiredness and loss of appetite. Untreated it can be fatal. Today the disease is becoming increasing rare in the Western world through a combination of antibiotic use and wide-spread immunisation. However, it still a significant health threat globally, and is one of the top ten causes of death world-wide. In particular it is one of the major killers of HIV positive patients, and although the number of cases year on year is slowly falling, there has also been a recent upsurge in reports of anti-biotic resistant strains. Therefore anything we can learn about its evolution could be extremely important in helping to combat the disease.

As we mentioned above modern human TB is caused a bacterium, Mycobacterium tuberculosis, but exactly when and how this bacteria first emerged has been something of a mystery. Today there are different strains of Mycobacterium found in lots of different species, including other mammals, reptiles, birds, even fish, however, genetic analysis had pegged the emergence of Mycobacterium tuberculosis at only 17,000 years ago. Given that TB it is endemic on both sides of the Atlantic, which last had significant contact far before this date, many had suspected this estimate might be a little bit too recent. However, previous evidence of ancient TB infections had been limited. DNA from Mycobacterium was found in a 9000 year old human skeleton, and in samples taken from of a Pleistocene bison, however finding evidence of TB in this ancient marine reptile would push its possible age back 245 million years.

800px-Plesiosaur_(9122845394)
Fossil reconstruction of a Plesiosaur. One of Proneusticosaurus’ more famous cousins. Source: Wikimedia Commons

Of course, given the age of these remains, it isn’t possible to recover DNA from the bacterium that might have infected this reptile. Therefore we’re just left with the physical evidence. The fossil remains are actually something of an enigma in of themselves. When they were discovered they were tentatively classified as belonging a new species called Proneusticosaurus silesiacus, however more recent investigation has suggested they probably belong a totally different taxon called Cymatosaurus. This confusion stems from the fact that Cymatosaurus is known only from skull fragments, whereas these fossils are all of the body with no skull associated with them. Without finding a skull and body fossils together it is can be hard to match the two up, and this wouldn’t be first time that the body and the skull of the same animal have briefly ended up with different scientific names. Either way we do know that this specimen belonged to a larger group called the Sauropterygia, which were a diverse group of swimming reptiles united by their use of four flippers to swim. The most famous members of this family are the Plesiosaurs.

So what about the physical evidence? In their paper the authors go through a process of elimination to try and discount other possible causes of these bleb-like bone lesions. The blebs are solid, ruling out scurvy, they’re too small to be associated with fungal infections, there’s no greater bone damage around them to indicate a fracture or physical injury of any kind, and developmental disorders are ruled out because those usually effect the deeper layers of the bone.  This latter argument also applies to bone cancers. That just leaves tuberculosis.

Even in modern cases these types of blebs are exceedingly rare. In general bone problems caused by TB infection are only seen in 1-2% of TB cases, with blebs being a tiny subset of these. For an overview of the incidence and diagnosis of modern bone tuberculosis here is a good paper from 2013. Without DNA evidence it is obviously impossible to be absolutely certain that this unfortunate reptile was infected with a Mycobacterium of some kind, but on the balance of probabilities it does seem that the best explanation for these lesions is that they were caused by TB, or by some infection extremely similar to it. This ancient pedigree would also help to explain why TB is seen in almost every modern animal group.

The final question of course is did the TB kill this particular animal? The authors point out that while it’s impossible to tell for certain in all likelihood this animal had lived with TB for some time before it died. It is possible that the effect on its mobility, and lung capacity, as the TB took hold would have fatal for a marine predator that would have relied on hunting for its food, but this is just speculation. Either way it is a fascinating glimpse into the antiquity of tuberculosis and opens the possibility of new, older, finds still to come.

 

Reference:

Surmik D, Szczygielski T, Janiszewska K, and Rothschild B. M., 2018. Tuberculosis-like respiratory infection in 245-million-year-old marine reptile suggested by bone pathologies. 5. Royal Society open science publishing. 

 

 

 

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